Solid oxide fuel cell (SOFC) assemblies typically operate at temperatures in the range of 500° C. to 1000° C. Seals are required between the margins of adjacent planar elements of the fuel cell stack to ensure separation of air and fuel through passages formed in such elements. Such seals must be stable for long periods of operating time at elevated operating temperatures. Some of such seals must be electrically insulative and thus typically are formed of glass ceramics, whereas other seals may be conductive and thus typically use silver/copper (Ag/Cu) braze alloys.
A problem in prior art SOFC assemblies is that the silver (Ag) in the Ag/Cu braze alloys migrate from braze joints into chemically and electrically active regions of the assembly wherein conductive braze can cause electrical shorting of adjacent elements or internal leakage between fuel and air passages. It appears that softened or molten braze tends to migrate in response to electric potential differences and/or spatial surface tension differences within a fuel cell stack. Electrochemical migration has been observed in other applications such as in thick-film conductors, as reported by Kim Vu in a published technical paper, “Silver Migration—The Mechanism and Effects on Thick-Film Conductors”, College of Engineering, Chemical and Material Science Engineering Department, San Jose State University (2003).
What is needed in the art is a method and apparatus for preventing braze migration in SOFC assemblies during high temperature use thereof.
It is a principal object of the present invention to prevent failure of SOFC assemblies from braze migration.